fix: disallow invalid x-amz-security-token for root credentials (#13388)

* fix: disallow invalid x-amz-security-token for root credentials

fixes #13335

This was a regression added in #12947 when this part of the
code was refactored to avoid privilege issues with service
accounts with session policy.

Bonus: 

- fix: AssumeRoleWithCertificate policy mapping and reload

  AssumeRoleWithCertificate was not mapping to correct
  policies even after successfully generating keys, since
  the claims associated with this API were never looked up
  properly. Ensure that policies are set appropriately.

- GetUser() API was not loading policies correctly based
  on AccessKey based mapping which is true with OpenID
  and AssumeRoleWithCertificate API.

cmd/iam.go

@@ -1817,7 +1817,7 @@ func (sys *IAMSys) GetUser(accessKey string) (cred auth.Credentials, ok bool) {
 
 	if ok && cred.IsValid() {
 		if cred.IsServiceAccount() || cred.IsTemp() {
-			policies, err := sys.policyDBGet(cred.ParentUser, false)
+			policies, err := sys.policyDBGet(cred.AccessKey, false)
 			if err != nil {
 				// Reject if the policy map for user doesn't exist anymore.
 				logger.LogIf(context.Background(), fmt.Errorf("'%s' user does not have a policy present", cred.ParentUser))

cmd/signature-v4-utils.go

@@ -149,6 +149,7 @@ func checkKeyValid(r *http.Request, accessKey string) (auth.Credentials, bool, A
 		// to retry with 503 errors when server is coming up.
 		return auth.Credentials{}, false, ErrServerNotInitialized
 	}
+
 	var owner = true
 	var cred = globalActiveCred
 	if cred.AccessKey != accessKey {
@@ -157,19 +158,25 @@ func checkKeyValid(r *http.Request, accessKey string) (auth.Credentials, bool, A
 		if !ok {
 			return cred, false, ErrInvalidAccessKeyID
 		}
-		claims, s3Err := checkClaimsFromToken(r, ucred)
+		cred = ucred
+	}
+
+	claims, s3Err := checkClaimsFromToken(r, cred)
 	if s3Err != ErrNone {
 		return cred, false, s3Err
 	}
-		ucred.Claims = claims
+
+	if len(claims) > 0 {
+		cred.Claims = claims
+
 		// Now check if we have a sessionPolicy.
-		if _, ok = claims[iampolicy.SessionPolicyName]; ok {
+		if _, ok := claims[iampolicy.SessionPolicyName]; ok {
 			owner = false
 		} else {
-			owner = cred.AccessKey == ucred.ParentUser
+			owner = cred.AccessKey == cred.ParentUser
 		}
-		cred = ucred
 	}
+
 	return cred, owner, ErrNone
 }
 

cmd/sts-handlers.go

@@ -772,6 +772,7 @@ func (sts *stsAPIHandlers) AssumeRoleWithCertificate(w http.ResponseWriter, r *h
 		subClaim:                   certificate.Subject.CommonName,
 		audClaim:                   certificate.Subject.Organization,
 		issClaim:                   certificate.Issuer.CommonName,
+		iamPolicyClaimNameOpenID(): certificate.Subject.CommonName,
 	}, globalActiveCred.SecretKey)
 	if err != nil {
 		writeSTSErrorResponse(ctx, w, true, ErrSTSInternalError, err)